Plant exhaust gases discharged from plants such as thermal plants or steel plants contain carbon dioxide (CO2) which brings about the greenhouse effect. Thus, to allow such plant exhaust gases to discharge into the atmospheric air is considered to be one of the causes of global warming.
As effective countermeasures against the global warming, a CCS (Carbon dioxide Capture and Storage) system that separates carbon dioxide from a plant exhaust gas and captures the separated carbon dioxide is known, for example. This system uses an absorbing liquid capable of absorbing carbon dioxide, and causes carbon dioxide contained in a plant exhaust gas to be absorbed by the absorbing liquid.
To be more specific, the carbon dioxide capture and storage system includes an absorption unit and a regeneration unit. The absorption unit brings the plant exhaust gas and the absorbing liquid into gas-liquid contact with each other, and causes carbon dioxide in the plant exhaust gas to be absorbed by the absorbing liquid. The regeneration unit heats the absorbing liquid having absorbed the carbon dioxide to cause the carbon dioxide to be released from the absorbing liquid, and captures the released carbon dioxide. The captured carbon dioxide is stored in the ground and so on. Thus, emission of carbon dioxide into the atmospheric air is reduced.
In order to efficiently absorb mild acidic carbon dioxide, alkaline amine solution is preferably used as an absorbing liquid. Thus, when a plant exhaust gas contains hydrogen sulfide (H2S), carbonyl sulfide (COS), carbon disulfide (CS2) and hydrogen cyanide (HCN) which are acid gases other than carbon dioxide, these acid gases can be also absorbed.
The plant exhaust gas from which carbon dioxide has been removed in the absorption unit is discharged, as an absorption-unit exhaust gas, from the absorption unit. When the absorption-unit exhaust gas is discharged from the absorption unit, an absorbing liquid component is entrained with the absorption-unit exhaust gas. Thus, there is concern over the diffusion of the absorbing liquid component into the atmospheric air, which exerts harmful effects on neighboring environments.
For this reason, a cleaning unit, which cleans the absorption-unit exhaust gas discharged from the absorption unit by means of cleaning water (e.g., deionized water), can be installed. In this case, the absorption-unit exhaust gas and the cleaning water make gas-liquid contact with each other, and the absorbing liquid component in the absorption-unit exhaust gas is captured by the cleaning water, whereby the absorbing liquid component can be removed from the absorption-unit exhaust gas. The cleaning water circulates in the cleaning unit, and the cleaning of the absorption-unit exhaust gas is continuously carried out. This cleaning unit may also clean a regeneration unit exhaust gas discharged from the regeneration unit.
However, since the cleaning water continuously cleans absorption-unit exhaust gas in the cleaning unit, the absorbing liquid component is accumulated in the cleaning water, which may result in lowering of cleaning effect of the cleaning water. Thus, a part of the cleaning water is continuously replaced with a new liquid (deionized water, etc.) for making up the cleaning water. In this case, the part of the cleaning water in which the absorbing liquid component is accumulated is extracted and discarded, the amount of which corresponds to the made-up cleaning water. Thus, an amount of a new liquid to be used for making up the cleaning water increases, as well as an amount of the cleaning water to be discarded increases, which invites increase in operating cost.
In order to handle these situations, there is known a method in which condensate water generated upon cooling of the regeneration unit exhaust gas discharged from the regeneration unit is used as the cleaning water. Namely, the condensate water can be used as an alternative to deionized water to be used as the cleaning liquid, whereby the amount of use of the deionized water can be reduced. In addition, since the regeneration unit exhaust gas contains carbon dioxide, the condensate water obtained therefrom is mild acidic to have a smaller pH value. Thus, a higher cleaning effect can be expected as compared with the cleaning by means of deionized water.
However, since the regeneration unit exhaust gas also contains the absorbing liquid component, the condensate water condensed from the regeneration unit exhaust gas contains an alkali absorbing liquid component. Thus, the pH in the condensate water comes close to neutral, which limits the improvement in capturing efficiency of the absorbing liquid component in the absorption-unit exhaust gas.